Recently, the demand for reflective displays that are bright, superior in color purity, and capable of easily performing full-color display with low power consumption has been increasing. For example, conventional light-emission type elements, such as CRTs, LCDs, PDPs and ELDs, have such characteristics that they are bright and easy to see, and therefore a number of technologies have been proposed. However, the above-mentioned light-emission type elements have a problem that they cause visual fatigue when viewed for a long time because emitted light needs to be directly looked at. Moreover, mobile devices such as mobile phones are often used outdoors and there is another problem that emitted light is offset under sunlight, resulting in deterioration in viewability. Meanwhile, among light-emission type elements, especially LCDs are growing in demand and are used for various display applications including large displays and small displays. However, LCDs have a problem that a viewing angle is narrow, and thus they have a problem in terms of viewability that should be improved in comparison to other light-emission type elements.
Meanwhile, although the amount of paper used for storing and conveying documents has been decreasing because of the widespread use of computers in offices, the tendency to print and read digital information in paper is still persistent when such information is perused. Therefore, the amount of paper that is temporally used and abandoned immediately after the use shows an upward trend in recent years, on the contrary. Moreover, the amount of paper that is consumed daily for books, magazines, newspapers and the like is seen as a threat in terms of natural resources and environment, and they do not seem to decrease unless the medium is changed. However, when the way of information recognition and the way of thinking by the human being are taken into consideration, the superiority of “paper” over “display” typified by CRTs (Cathode Ray Tubes: Braun tubes) and transmission type liquid crystal displays cannot be ignored.
Therefore, the electronic paper in which the merit of paper and the merit of displays, which can directly handle digital information, are combined has been recently expected to be put into practical use as an electronic alternative to paper. The characteristics that the electronic paper is required to have include being a reflection type display element, having high reflectivity to white light and high contrast ratio, being capable of displaying with high definition, having a memory effect in display, being capable of driving with a low voltage, being thin and light, being inexpensive, and so on.
The display systems of electronic paper include a reflection type liquid crystal system, an electrophoresis system, a two-color ball system, an electrochromic (hereinafter, sometimes abbreviated as EC) system. Examples of the reflection type liquid crystal system includes a G-H type liquid crystal system using dichromatic pigment, a cholesteric liquid crystal system, and so on. This reflection type liquid crystal system has an advantage that it does not need to use a backlight and thus consumes smaller electrical power in comparison to the existing light-emission type liquid crystal system. However, it involves the dependence on viewing angle and has low light reflectivity, and thereby has a problem that the screen inevitably becomes darker.
The electrophoresis system exploits a phenomenon called “electrophoresis” in which white pigments, black toner, or the like moved onto electrodes by the effect of an electric field. The two-color ball display system involves a spherical body painted with two colors such as white and black in a half-and-half fashion, and uses the rotation by the effect of an electric field. Both the systems have a merit that they consume low electrical power and that they do not involve the dependence on viewing angle. However, it is believed that these systems cannot achieve a high contrast because they require gaps large enough for allowing particulate bodies to enter therein, which makes closest packing difficult. Moreover, when it is to be displayed in full color, a color juxtaposition method using a color filter is adopted, posing a problem that the reflectivity decreases and the screen inevitably becomes darker.
Meanwhile, the EC system is a system in which a reversible oxidation-reduction reaction is caused by an application of an electric field and color development/color disappearance caused by the reaction is exploited. EC display elements have heretofore been used in dimming mirrors of automobiles, clocks, and so on. The display by such an EC display element does not require a polarizing plate and the like, does not involve the dependence on viewing angle, is a light reception type and thus superior in terms of viewability, has a simple structure, and is easily constructed in a large size. Moreover, it has another merit that light emission of various color tones is possible by selecting proper materials.
To show display in full color in an EC display element, there is known a method that uses pigments capable of coloring including cyan (hereinafter, sometimes abbreviated as C), magenta (hereinafter, sometimes abbreviated as M), and yellow (hereinafter, sometimes abbreviated as Y), which are used in subtractive color mixture, and that forms a structure having a C-coloring layer, an M-coloring layer and a Y-coloring layer in a parallel arrangement or in a laminated arrangement. This can afford a display device capable of coloring in full color. For example, black can be displayed by mixing colors of C, M and Y. Moreover, white can be displayed by bringing each pigment to a decolored state while the background color is white. Since the EC display element is a reflection type display element in which coloration/decoloration can be electrically repeated without using a color filter as described above, they are superior to other display systems in terms of burden put on eyes and in terms of contrast.
Research of a material called a π-electron conjugated macromolecule as one of the materials that constitutes the above-mentioned coloring layer has been progressing. There are various π-Electron conjugated polymers including polyacetylene, polypyrrole, polyaniline, polyparaphenylenevinylene, and polythiophene, and they are promising as materials that constitute polymer light-emitting diodes (film displays), solid state lightings, organic photoelectric cells, memory devices, organic field effect transistors, printing electronics, conductors, lasers, sensors, solid capacitors, and so on. Among such π-electron conjugated polymers, polymers exhibiting electrochromic properties are known. It is supposed that in order to obtain an EC element capable of showing colors in full color by the aforementioned coloration/decoloration of C, M, and Y, the electrochromics of a π-electron conjugated macromolecule must change from colored states to colorless states of C, M, and Y, respectively. However, the electrochromic properties of most of common π-electron conjugated polymers exhibit color change between colored states, and there are a very few materials exhibiting color change from a colored state to a colorless state as described above.
Poly(ethylene-3,4-dioxythiophene) is known as a typical materials which change in color from a colored state to a approximately colorless state. However, this material is a π-electron conjugated macromolecule which changes in color from a navy blue colored state close to C to a pale blue decolored state and no materials which change in color from M or Y to a colorless state have been known.
Patent literature 1 discloses a method for producing a monomer compound such as 1H-thieno[3,4-d]imidazol-2(3H)-one. However, nothing about a monomer compound in which two molecules of 1H-thieno[3,4-d]imidazol-2(3H)-one or the like are linked via an aromatic compound or the like is disclosed and nothing about a polymer to be obtained using the same and the electrochromic properties of this polymer is also disclosed.
Patent literature 2 discloses a polymer having 1H-thieno[3,4-d]imidazol-2(3H)-one or the like as a constitutional unit, and a copolymer having 1H-thieno[3,4-d]imidazol-2(3H)-one or the like and phenylene or the like as constitutional units. However, nothing about a monomer compound in which two molecules of 1H-thieno[3,4-d]imidazol-2(3H)-one or the like are linked via an aromatic compound or the like is disclosed and nothing about a polymer to be obtained using the same is also disclosed.